The most familiar and widely utilized electrostatic printing technique is that of xerography wherein latent electrostatic images formed on a charge retentive surface, such as a roller, are developed by suitable toner material to render the images visible, the images being subsequently transferred to an information carrier. This process is called an indirect process because it first forms a visible image on an intermediate surface and then transfers that image to an information carrier.
Another method of electrostatic printing is one that has come to be known as direct electrostatic printing. This method differs from the aforementioned xerographic method in that charged pigment particles (in the following called toner) are deposited directly onto an information carrier to form a visible image. In general, this method includes the use of electrostatic fields controlled by addressable electrodes for allowing passage of toner particles through selected apertures in a printhead structure. A separate electrostatic field is provided to attract the toner particles to an information carrier in image configuration. The novel feature of direct electrostatic printing is its simplicity of simultaneous field imaging and particle transport to produce a visible image on the information carrier directly from computer generated signals, without the need for those signals to be intermediately converted to another form of energy such as light energy, as is required in electrophotographic printers, e.g., laser printers.
U.S. Pat. No. 5,036,341, granted to Larson, discloses a direct printing method which begins with a stream of electronic signals defining the image information. A uniform electric field is created between a high potential on a back electrode and a low potential on a toner carrier. That uniform field is modified by potentials on selectable wires in a two dimensional wire mesh array placed in the print zone. The wire mesh array consists of parallel control wires, each of which is connected to an individual voltage source, across the width of the information carrier. The multiple wire electrodes, called print electrodes, are aligned in adjacent pairs parallel to the motion of the information carrier; the orthogonal wires, called transverse electrodes are aligned perpendicular to the motion of the information carrier. All wires are initially at a white potential V.sub.W preventing all toner transport from the toner carrier. As image locations on the information carrier pass beneath wire intersections, adjacent transverse and print wire pairs are set to a black potential V.sub.b to produce an electrostatic field drawing the toner particles from the toner carrier. The toner particles are pulled through the apertures formed in the square region among four crossed wires (i.e., two adjacent rows and two adjacent columns), and deposited on the information carrier in the desired visible image pattern. The toner particle image is then made permanent by heat and pressure fusing the toner particles to the surface of the information carrier. A drawback in the method of U.S. Pat. No. 5,036,341 is that during operation of the control electrode matrix, the individual wires can be sensitive to the opening or closing of adjacent apertures, resulting in undesired printing due to the thin wire border between apertures. That defect is called cross-coupling.
U.S. Pat. No. 5,121,144 discloses a control electrode array formed on an electrically insulating substrate with a plurality of apertures arranged therethrough, one ring-shaped electrode surrounding each of those apertures and one connector joining each ring-shaped electrode to its associated control voltage source. The apertures and associated ring electrodes are arranged in parallel rows and columns on the insulating substrate. The rows extend transversely across the width of the array, i.e., perpendicular to the motion of the information carrier. The columns are aligned at a slight angle to the motion of the information carrier in a configuration that allows printing to be achieved in sequence through each transverse row of apertures as the required dot positions arrive under the appropriate passage, thereby also allowing a larger number of dots to be deposited in a transversal direction on the information carrier. This results in a substantially enhanced printing performance, and a considerably reduced cross-coupling, since every aperture is not surrounded by any other control electrode than the intended.
However, since the apertures and their associated ring electrodes are arranged in parallel rows, the connectors leading to a ring electrode of one particular row may intersect one or more other rows. Thus, several connectors might be arranged in the relatively narrow space between two adjacent apertures of a row.
For instance, as four parallel rows of ring electrodes, each of which being individually connected to a control voltage sources, are aligned on the array, the connectors leading to the fourth row necessarily pass through the first, second and third rows. Similarly, the connectors leading to the third row extend through the first and second rows and the connectors leading to the second row extend through the first row. Consequently, three connectors extend on each side of every aperture of the first row, two connectors extend on each side of every aperture of the second row, and one connector extends on each side of every aperture of the third row. Since the distance between two adjacent apertures of a same row is typically less than one millimeter, the connectors extending between two adjacent apertures may substantially influence the field configuration about those apertures. For instance, when black voltages V.sub.b are simultaneously applied to "open" a first aperture located on one row and a second aperture located on another row, if the connector leading to the ring electrode of said second aperture borders on said first aperture, it has been observed that the toner particles attracted through that first aperture tend to be slightly deflected from their initial trajectory, due to their interaction with the electric field generated by the bordering connector. As a result, the dots addressed through said first aperture are slightly displaced with respect to the central axis of that aperture. This defect is known as the dot deflection phenomenon. Thus, to improve the print quality of direct electrographical printing devices, it is essential to reduce cross coupling and uncontrolled dot deflection. Accordingly, there is still a need for a method to reduce undesired interaction between adjacent electric fields on the array.